Cell Division

Cell division is an integral part of the process of reproduction, growth, and repair of the body. During cell division, genetic material is distributed among the daughter cells. This is the important phenomenon taking place during all types of cell divisions. There are two types of cell division observed in eukaryotic cells. They are mitosis and meiosis. The distribution of genetic materials among the daughter cells is so important because their future development and generation of cells developing from that will be entirely dependent on the genetic material that they receive from the parent cell. Mitosis is the cell division that occurs in the somatic cells (body cells) and meiosis takes place in the sex organs for the production of gametes. The main features and the process of mitosis and meiosis are discussed below.

Mitosis

Mitosis is the process that facilitates the equal partitioning of replicated chromosomes into two identical groups. Two new daughter cells arise from one original cell. All the cells created through mitosis are genetically identical to one another and to the cell from which they came. The main purpose of mitosis in eukaryotic cells is:

• Growth of the individual,
• To repair tissue, and
• To reproduce asexually.
  

Mitosis is a nuclear division in which the daughter cells receive the same number of chromosomes as that of the parent cell. The nuclear division is sometimes referred to as karyokinesis, which is followed by the cytoplasmic division known as cytokinesis. The daughter cells resulting from mitosis are identical to each other and also to the parent cell in the quantity and quality of genetic material. The genetic information, which the cell is copying and distributing during mitosis, is contained in the form of chromosomes.

The period between two successive cell divisions is referred to as the interphase.It is not a part of mitosis, but forms the preparatory stage for cell division. The main metabolic activities during this stage of cell are respiration, protein synthesis, and the duplication of genetic material (DNA replication). Cells usually are small with no large vacuoles.

The interphase stage is subdivided into three parts: Gl, S, and G2. In the Gl stage, the cell carries out its "housekeeping" functions while it collects the materials it will need to divide. S stands for DNA synthesis and this is the stage in which the DNA makes a copy of itself. An enzyme (DNA polymerase) assists each DNA double helix in making a copy of it in a process known as DNA replication. The two identical DNA double helices are represented in the chromosomes as the sister chromatids, and they are held together by the centromere, which is visible as a constricted area somewhere along the length of the chromosome. The G2 stage of mitosis comes next, and in this stage the cell checks to see that everything is ready to begin mitosis. When the cell has duplicated its DNA and checked it to make sure that there are no errors, it is ready to begin the distribution of the DNA to two separate cells.

Mitosis is divided into four stages-prophase, metaphase, anaphase, and telophase. The important changes that the cells undergo in each phase are described below.

Prophase

During prophase the nuclear membrane and nucleolus disappear. The chromosomes themselves condense from long, thin filaments into compact rods so that they can be more easily moved. The apparatus needed to move the chromosomes around is set up. In animal cells this is done by the two centrioles, which move toward opposite poles of the cell and begin to form the mitotic spindle. Plant cells do not have centrioles; they use a different mechanism to form the spindle fibers. Each chromosome has a spindle fiber attached to it at the centromere and extending off to either side of the cell where the centrioles are located. These spindle fibers are exerting tension on the chromosomes and when the centromere splits later on, this will allow the chromosomes to be pulled to either end of the cell.

Metaphase

Metaphase is an easy stage to recognize because all of the chromosomes are lined up at the equator of the cell. Depending on where the viewer is "standing," it can look as though the chromosomes are lined up east to west or it can look as though the chromosomes are aligned north to south.

Anaphase

In anaphase the centromere on each chromosome splits. Because the spindle fibers are exerting tension on the chromosomes, when the centromere splits each chromatid is pulled toward the spindle pole that it faces. Once a chromatid has its own centromere it can be called a chromosome.

Telophase

In telophase the chromosomes reach the opposite poles of the cell and their attached spindle fibers disappear. A new nuclear envelope forms, the nucleolus reappears, and the condensed chromosomes expand once more. The cytoplasm divides in a process called cytokinesis, and this forms the two daughter cells. In animal cells the cell membrane pinches in from either side by a constriction and separates into two. In a plant, a new cell wall begins to form in the middle of the cell and gradually grows longer from each end as it works its way toward the edges of the cell.

Mitosis is finished and there now are two cells, which are identical to each other, and identical to the cell from which they came.

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